Aqueous solutions of partially and fully hydrolyzed polyvinylamines have great utility in improving paper dry strength, retention and drainage, contaminant control, and application efficiency with other additives, i.e. —starch, sizing, and defoamer. These positive effects are most noticeable in recycled containerboard grades, but can generally be observed in all paper and board grades. Polyvinylamines are highly effective for these purposes, and are enjoying extensive commercial use. However, polyvinylamine chemistry is quite expensive to produce. A greener product is desired which will retain the same functionality of a polyvinylamine homopolymer but that can be manufactured at a lower cost with lower environmental impact.
Polyvinylamines are typically made by solution free-radical polymerization of N-vinylformamide monomer followed by base hydrolysis. The products are usually in a aqueous form at an active polymer solids of about 10-20% by dry weight. Polyvinylamine is highly cationic in solution due to its high density of primary amine or amidine functionality. In general, a polyvinylamine product is used as a single component for papermaking at the wed end.
U.S. Pat. No. 4,940,514 discloses utility of a blend of enzymatically digested starch and polyvinylamine, poly-DADMAC, or poly-vinylimidazoline as a paper strength agent. The claims require the starch to be enzymatically reduced and to be within a specified solution viscosity. They also specify that the ratio of cationic polymer to starch be 1 to 20 parts polymer to 100 parts starch. US patent application 20040112559 discloses blends of low viscosity starch and synthetic polymers such as polyacrylamides and polyvinylamines. The starches used are all enzymatically degraded and have low viscosity. There are no synergetic effects in those blends.
US patent application 20050109476 discloses utility of increasing starch adsorption in paper by co-extruding starch with polyvinylamine. The mixture has to be passed through an extruder. U.S. Pat. No. 6,616,807 teaches reacting polyvinylamines with starch. The reaction requires the addition of the polyvinylamine to the starch above its gelatinization temperature. It also claims polyvinylamines as starch retention aids. In this latter case a separate addition to papermaking stock is employed.
U.S. Pat. No. 7,074,845 discloses blends of swollen, unruptured starch granules, anionic latexes, and optionally anionic or cationic co-additives including polyvinylamine or poly-DADMAC. Carboxylmethyl cellulose (CMC) appears preferred as a co-additive from the examples. In this case, the starch is not completely cooked, and anionic latex must also be present in order to practice the invention. U.S. Pat. No. 6,746,542 teaches that prior art reacting polyvinylamines with starch improved paper strength, but resulted in unacceptable reductions in production rate. The improvement is a two-component addition of polyvinylamine or other low molecular weight “cationizer” and a “drainage aid” to the starch, again above the gelatinization temperature. The drainage aid is selected from several cationic or nonionic polymers of greater than 1 million in molecular weight.
A number of prior art references were cited in U.S. Pat. No. 6,746,542. They teach addition and reaction of a synthetic polymeric component to starch. All require the addition by “heating”, “digesting” or “reacting under alkaline conditions” to gelatinize the starch. None teach a simple, stable aqueous blend of a high solids and high viscosity starch solution and polyvinylamine that can be formed at ambient temperature and neutral pH.
U.S. Pat. No. 7,090,745 teaches production of hydrogels by reaction of polyvinylamines with reducing sugars. The scope of 7,090,745 includes polymeric sugars such as starch and cellulose, although all of the examples use monomeric sugars. The hydrogels are useful as paper strength agents. The hydrogels are created by blending polyvinylamine and a reducing sugar at room temperature, then heating and mixing the blend for a period of time. The hydrogels are water insoluble materials and not dispersible in water.
US patent application 20050022956 teaches an improved surface sizing composition including a sizing agent (typically starch), a cationic polymer including polyvinylamine, and an anionic polymer such as SMA. The anionic polymer must be present to practice the claimed invention.
There remains a need to develop a lower cost, more environmentally friendly, polyvinylamine based dry strength product for papermaking application. The product must be equivalent or better than polyvinylamine on weight active basis as a dry strength resin and a drainage aid for recycled linerboard and other paper products. The inventors have surprisingly found that the compositions of polyvinylamine with liquid cationic starches at a certain ratios of polyvinylamine to starch show a synergistic effect in papermaking applications and exhibit improved dry strength and drainage properties compared to polyvinylamine alone on the same weight active basis. It reduces the cost-in-use by about 20%. The blend is stable and does not suffer from starch retrogradation in storage.